Move SubIterator handling out of Base

graph/memstore/all_iterator.go

@@ -31,6 +31,11 @@ func NewMemstoreAllIterator(ts *TripleStore) *AllIterator {
 	return &out
 }
 
+// No subiterators.
+func (it *AllIterator) SubIterators() []graph.Iterator {
+	return nil
+}
+
 func (it *AllIterator) Next() (graph.Value, bool) {
 	next, out := it.Int64.Next()
 	if !out {

graph/memstore/iterator.go

@@ -105,6 +105,11 @@ func (it *Iterator) Next() (graph.Value, bool) {
 	return graph.NextLogOut(it, it.Last, true)
 }
 
+// No subiterators.
+func (it *Iterator) SubIterators() []graph.Iterator {
+	return nil
+}
+
 func (it *Iterator) Size() (int64, bool) {
 	return int64(it.tree.Len()), true
 }